The present invention relates generally to gypsum board and methods for making gypsum board. In particular, the present invention relates to an efficient and economical method for producing gypsum board that possesses antimicrobial (e.g., antibacterial and antifungal) properties.
Gypsum board, also known as drywall and wallboard (hereinafter, wallboard), is a common building material. It is used in a variety of construction applications. Some of the more common uses for wallboard include the construction of interior walls, partitions, and ceilings. It is a popular construction material because it possesses desirable mechanical and aesthetic properties. They are durable, economical, and fire-retardant. Wallboard also provides excellent compressive-strength properties with a relatively low density.
Perhaps most important for interior applications, they are easily decorated by either paint or wallpaper and are therefore attractive as surfacing materials.
In general terms, wallboard is a solidified mineral (gypsum) that is sandwiched between two thick pieces of paper. Gypsum is a mineral (CaSO4Âxc2x7H2O) that may be mined from the ground as a rock or produced synthetically as a byproduct from smokestack environmental control devices. The following paragraphs outline a typical method for making wallboard.
Natural gypsum mined from the ground is shipped to the plant and stored in a rock pile until needed. The gypsum rock is then prepared by grinding it into small pieces followed by drying it in a kiln. The dry gypsum is then run through a roller type crushing mill where it is ground into a fine powder called xe2x80x9cland plasterxe2x80x9d.
The land plaster is then heated to remove about three-quarters of the water that is chemically bound in the gypsum. The result is a very dry powder called xe2x80x9cstuccoxe2x80x9d that when mixed with water, quickly rehydrates and xe2x80x9csets-upxe2x80x9d or hardens. The stucco is then stored in large silos to await use in the wallboard manufacturing process.
From the silo, the stucco enters the wet end of the manufacturing process. The stucco is blended with water and other ingredients, depending upon the type of wallboard being made, to make a slurry or paste. The slurry is spread on a long, moving stream of cream-colored paper that travels on a conveyor belt. The slurry is then covered or xe2x80x9csandwichedxe2x80x9d with a top paper. This long sheet of sandwiched gypsum paste will travel between 200 and 2000 feet on the conveyer to a cutting station. The conveyors usually run at a speed that provides about a 4 to 5 minute transit time to the cutting station. This time is needed to allow the gypsum paste to harden before it is cut. Once it reaches the cutting station it is cut into desired lengths. The cut wallboard panels are then turned cream side up and placed in a kiln to dry.
In many wallboard production processes, starch, such as that manufactured by Archer Daniels Midland Company (ADM), is added to the gypsum core material at some point during the manufacturing process. Its role is to keep the paper attached to the gypsum core. Although it is commonly believed that starch acts as an adhesive that bonds the paper to the gypsum core, ADM technical material states that the starch actually serves to protect the gypsum crystals that form the bond between the gypsum core and the paper during the drying process. Regardless of the actual mechanism by which starch works to keep the paper attached to the gypsum core, starch is present at the interface between the paper and the gypsum core and its presence at that interface is one of the factors that underlie the present invention.
One of the drawbacks to using traditional wallboard products is their susceptibility to moisture absorption in damp environments. This is one reason wallboard is usually used only for interior construction. Unfortunately, products used in interior construction sometimes can encounter water due to leaks in roofs, windows, or plumbing. Furthermore, many geographical areas are characterized by high humidity which also provides a source of water that can be absorbed by wallboard. Once exposed to moisture, traditional gypsum wallboard products are susceptible to supporting microbial growth, specifically fungal and bacterial growth.
Wallboard is susceptible to supporting microbial growth because it provides growth conditions suitable for microbial growth. In addition to warm, moist environments, microbes usually need a readily available source of nutrients to grow. Starch, such as that found at the interface of the paper and the gypsum core, can serve as a nutrient for microbial growth.
The growth of fungus and bacteria on wallboard is undesirable for many reasons. First, it traps moisture in the wallboard which leads to structural weakening and promulgation of even more fungus and bacteria. Unpleasant odors and staining are also associated with microbial growth. More seriously, many people are susceptible to life threatening allergic responses when exposed to fungal spores. The issues created by microbial growth on wallboard, especially the human health issues, drives a continuing need for wallboard that is resistant to microbial grown.
The patent literature contains several examples of attempts to address the problem of microbial growth on wallboard. To date, these attempts have failed to provide an economically viable solution to the problem. For example, U.S. Pat. Nos. 3,918,981 and 3,998,944 to Long and assigned to U.S. Gypsum Company discuss the application of a fungicidal agent to the paper that covers the gypsum core. The fungicidal agents discussed therein are water-insoluble metal quinolinolate salts, more specifically, a copper quinolinolate. Such biocides are undesirable from an environmental perspective. Furthermore, the antifungal compositions discussed in the Long patents are quite specific in their application and lack the flexibility needed to handle the array of applications for gypsum wallboard.
Similarly, recently published U.S. applications US 2003/0031898; US 2003/0035981; and US 2003/0037502 attempt to address the problem of fungus growth on wallboard. The xe2x80x3898 and xe2x80x3981 documents attempt to solve the problem by adding a large amount of active ingredient to the wallboard. The examples provided in both documents add antimicrobial agents directly to the gypsum slurry at levels approaching 5000 ppm based upon dry weight of the gypsum in the wallboard. Using such a high level of active ingredient in a wallboard process is not commercially desirable for a number of reasons, costs being the primary consideration. The toxicity of the preferred active ingredient used in the xe2x80x3898 and xe2x80x3981 documents is another drawback to its use.
In addition, to the extent the xe2x80x3898 and xe2x80x3981 documents discuss treating the paper rather than the gypsum core, the xe2x80x3898 and xe2x80x3981 documents discuss either spraying the finished paper or adding the active ingredient during manufacture of the paper (i.e., to the paper pulp). Spraying, as discussed in the xe2x80x3898 and xe2x80x3981 documents, can be difficult and costly as it usually requires either additional equipment or steps to the manufacturing process or both. Spraying a surfactant based liquid, such as the liquid that carries the xe2x80x3898 and xe2x80x3981 active ingredient, often leads to foaming problems which lead to non-uniform application and can disrupt manufacturing processes.
Similarly, adding any extra ingredient to the paper pulp is usually undesirable due to the fact that paper processes are finely tuned and easily disturbed. Paper manufacturers tend to avoid any unnecessary changes to well functioning processes. If an active ingredient is incorporated via addition to the paper pulp, the active ingredient must normally be present in high concentrations to have efficacy at the surfaces of the paper where it is needed. Furthermore, if active ingredient is added to the paper pulp, the active ingredient must attach itself or be attracted to the paper fibers (i.e., have substantivity to the paper fibers) otherwise the active ingredient will wash away from the pulp as the water is pulled from the paper slurry. Not only is this wasteful but it also causes wastewater treatment problems. Furthermore, active ingredients that are attracted to the paper fibers show poorer efficacy because of their lack of mobility.
The xe2x80x3502 document addresses the wallboard/microbial growth problem in a different way. The xe2x80x3502 document replaces the paper coverings of the wallboard with polymeric fibrous sheets and attempts to remove most if not all microbial nutrients (e.g., starch) from the gypsum core. The xe2x80x3502 document notes that such an approach has had trouble finding commercial acceptance.
The present invention derives from research directed at developing a commercially viable process for making a wallboard that exhibits antimicrobial characteristics. One result of this research was a wallboard which exhibits antimicrobial characteristics and resists the growth of microbes. The wallboard according to the invention comprises a gypsum core having at least a first face and a non-woven covering in contact with that face.
The wallboard also comprises an antimicrobial system having at least a first antimicrobial agent. The antimicrobial system utilized in the practice of the invention may also have a second antimicrobial agent depending upon the manner in which the antimicrobial system is applied to the wallboard. In one embodiment the antimicrobial system is a non-foaming antimicrobial system comprising a first antimicrobial agent in a first carrier and a second antimicrobial agent in a second carrier where the two carriers are soluble in each other. Furthermore, the first and second antimicrobial agents are present in the wallboard, or a component thereof, at levels sufficient to exhibit efficacy against microbes.
The invention also encompasses a method for producing wallboard that exhibits antimicrobial characteristics and resists the growth of microbes. The method according to the invention comprises adding an antimicrobial system to the wallboard or to a component thereof at levels sufficient to exhibit efficacy against microbes. The step of adding an antimicrobial system to the wallboard may comprise adding a non-foaming antimicrobial system to the paper coating of the wallboard, to the gypsum core of the wallboard, or both. The step of adding an antimicrobial system to the wallboard may also comprise adding individual antimicrobial agents to the slurry that forms the gypsum core.
The preferred antimicrobial agents that may be used the practice of the invention include propiconazole, sodium pyrithione, tolyl diiodomethyl sulfone; tebuconazole; thiabendazole; 3-iodo-2-propynyl butylcarbamate; and mixtures thereof.
Accordingly, the invention also encompasses an antimicrobial composition for imparting antimicrobial characteristics to a substrate. The composition according to the invention comprises a first antimicrobial agent selected from the group consisting of propiconazole, sodium pyrithione, and mixtures thereof; and a second antimicrobial agent selected from the group consisting of tolyl diiodomethyl sulfone; tebuconazole; thiabendazole; 3-iodo-2-propynyl butylcarbamate; and mixtures thereof.